Progress in the hundred billion dollar semi-conductor industry depends, in part, on the ability to mark (i.e. write, dope or etch) a surface with small features at controlled separations. The current limit is the making of marks separated by a few tenths of a nanometre (commonly 0.3 microns, i.e. 3,000 .ANG., which is roughly one thousand atoms separation). Patterns of these dimensions constitute the lower limit of what can be achieved by the conventional method of marking, which involves the use of a patterned mask to shield portions of the surface from the agent (electrons, light or chemicals) used in order to mark the surface. It has not proved possible to make patterned masks having features smaller than tenths of a micron. Moreover, masks with such small features already suffer from irreproducibility.
U.S. Pat. No. 5,645,897 issued to Andra discloses a method for surface modification by ion bombardment of the surface or the region in front of the surface portion being etched or coated. The ion source is chosen to produce ions which are highly charged and possessing kinetic energies sufficiently high to permit the ions to approach the surface but low enough to prevent penetration of the surface. A stated advantage of the process of this patent is that the highly charged state of the ions and their low kinetic energies results in very localized energy deposition thereby giving rise to improved spatial resolution in the imprinting of patterned masks for etching or coating the surface. This patent also discloses combining the feature of localized energy deposition using the ion beams with conventional lithographic masking techniques for producing precise etching patterns.
U.S. Pat. No. 5,405,481 issued to Licoppe et al. is directed to a gas photonanograph device for production of nanometre scale surface patterns. The device includes a head comprising a fiber optic cable terminating in a tip and microcapillary channels also terminating at the tip that feed reactive gas from a gas reservoir. The tip is spaced from the area of the substrate surface being light activated. Nanopatterns can be produced by scanning this device, as one might write with a pen, the tip of the pen here being a focussed light source.
U.S. Pat. No. 4,701,347 issued to Higashi specifically mentions the photolysis of molecules adsorbed on a surface as a method for growing patterned metal layers on semiconductor. However, in common with earlier patents cited therein, going back to U.S. Pat. No. 3,271,180 issued on Sep. 6, 1966, the pattern of photolytic and thermal reaction induced by illumination of the adsorbate derives from the presence of a mask between the light source and the adsorbed layer.
U.S. Pat. No. 5,322,988, in common with U.S Pat. No. 4,701,347 referred to above, uses laser irradiation to induce photochemical and thermal reaction between an adsorbate layer and the underlying substrate, but the reaction etches rather than writes (the etching is termed "texturing"). Reaction, it is stated, only occurs where the laser is impinging with sufficient fluence, i.e. patterned illumination (as beneath a "mask") is the source of patterned etching.
D. J. Ehrlich et al. in Appl. Phys. Lett. 36, 698 (1980) describe a method of mask-free etching of semiconductors based on the ultraviolet photolysis of gaseous methyl halides. The place of the patterned mask is taken by an interference pattern, i.e. it derives, once more, from patterned irradiation of the surface.
U.S. Pat. Nos. 4,608,117 and 4,615,904 issued to Ehrlich et al., disclose maskless growth of patterned films. This method describes a two-step process. In step one a pattern is written on the surface using a focussed light-beam or electron-beam as a pen, and photodissociation as the agent for writing (the ink). Once a 1-2 monolayer pattern of metal or semiconductor has been written in this fashion, step two involves uniform irradiation of the gaseous reagent and the surface which results in the accumulation of material on the "prenucleated sites", i.e. in the close vicinity of the pattern of deposition formed in step one. Consequently this second growth-phase is mask-free. In the mask-free film-growth phase "atoms are provided dominantly by direct photodissociation of the gas-phase organometallic molecules." (U.S. Pat. No. 4,608,117, column 2, lines 12 and 13). Film growth, it is stated, occurs selectively in the prenucleated regions where impinging atoms originating in the gas phase have a higher sticking coefficient at the surface.
U.S. Pat. No. 5,129,991 issued to Gilton describes an alternative scheme for mask-free etching. An adsorbed etch-gas (a chloride or fluoride) is present on a substrate which has macroscopic regions fabricated from different materials having different photoemission threshold-values for the release of electrons. This substrate is illuminated with a wavelength of light selected to give electron emission from some regions but not from others. The emitted electrons cause etching to occur only on those regions of the substrate which are composed of materials with a low enough photoemission threshold to emit electrons; i.e., reaction is localised, but localised to macroscopic areas.
C. Yan et al., J. Phys. Chem., 99 6084 (1995), have reported that molecular chlorine impinging as an energetic (0.11 eV) beam of molecules on a Si(111) 7.times.7 substrate reacts directly from the gas to halogenate the substrate preferentially at surface silicon-atom sites which are adjacent to one another (70% adjacent, 30% non-adjacent). Though these chlorinated pairs of sites recur randomly across the surface, they constitute short-range order, i.e., a simple form of molecular-scale patterning.
It is well known that one can produce a pattern on a surface by adsorbing a weakly-bound layer of molecules which, in their most stable configuration, form a pattern. The existence of such adsorbate patterns has been shown incontrovertibly by Scanning Tunnelling Microscopy (STM) which reveals the locations and separations of the adsorbate molecules (refs. 1-4). The origin of these spontaneously-formed molecular patterns has been the subject of theoretical analysis (e.g., refs. 5-7).The patterns are governed by the effect on the adsorbate of the regular arrangement of the atoms in the underlying crystal (termed the "substrate"), by the size and shape of the adsorbed molecules themselves which can interact with one another to form an adsorbate pattern due to that cause alone, and by the coverage which determines the inter-adsorbate separation and hence affects the favoured pattern of adsorption. Through the choice of these variables (substrate, adsorbate and coverage) one obtains differing patterns. These adsorbate patterns repeat at intervals as small as a few atomic diameters.
Typically, what are termed `adsorbate layers` have heats of adsorption of 0-1 eV. At the low end of this range they are said to be `physisorbed`, and at the high end `chemisorbed`. They are adsorbate layers by virtue of the fact that they are subject to desorption, with a heat of desorption corresponding to the prior heat of adsorption, by warming the surface.
It would be advantageous to exploit the existence of these molecular-scale adsorbate patterns as a means to mark the underlying surface. This would provide an avenue to mask-free marking and, more importantly, to marking at separations at least a hundred times less than the lowest limit achievable through the use of the present procedures employing masks.